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Translocation
Translocation is the term given to the
movement of assimilates up and down a
plant.
These assimilates are usually sucrose and
other chemicals produced by the plant.
Sugars are transported in the phloem
tissue.
Source- A part of a plant which releases
and loads sucrose into the phloem.
Sink- Part of the plant which removes
phloem.
During the Summer, sucrose is usually
made in the leaves, where it is a product
of photosynthesis. Therefore the leaves
are the source. Then the sucrose is
transported to the roots where it is stored.
The roots are therefore the sink.…read more

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How Sucrose Enters The Phloem
Sucrose is loaded into the phloem by
an active process.
The companion cells uses ATP to
actively transport H+ ions out of the
cells into surrounding tissue.
This creates a diffusion gradient of H+
ions between the companion cells and
the cells surrounding them. The
hydrogen ions then diffuse back into
the companion cells.
This diffusion happens through special
proteins called co-transporter proteins.
These proteins allow the hydrogen
ions to bring sucrose with them as
they enter the companion cells.
As the concentration of sucrose
increases inside the companion cells,
they diffuse into the sieve tube
elements through the plasmodesmata
=Movement of sucrose at source…read more

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Movement of Sucrose Along Phloem
At The Source
Sucrose entering the sieve tube
element reduces the water
potential inside the sieve tube. As
a result, the water follows by
osmosis from surrounding cells.
This increase hydrostatic pressure
in the sieve tube at the source.
At The Sink
Sucrose many be needed in the cells
surrounding the phloem. The
sucrose may be converted into
starch for storage or used for a
metabolic process. The means
that the concentration of sucrose
is low in these cells. The sucrose
is then transported into these cells
by diffusion or active transport.
This increases the water potential
in the phloem to increase causing
the water to leave the sieve tube.
This reduces hydrostatic pressure
in the sink.…read more

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Along The Phloem-Continued
Water entering the phloem at the source, moving
down the hydrostatic pressure gradient and
leaving the phloem at the sink produces a flow
of water along phloem. This flow carries sugars
and other assimilates along the phloem. This is
called mass flow. It can occur up or down the
plant depending on where it is needed.…read more

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Evidence For This Mechanism
How We Know The Phloem Is Used
If a plant is supplied with radioactively labelled carbon dioxide carbon dioxide, this labelled
carbon soon appears in the phloem.
When phloem is removed on a tree, the area above the cut swells as sugar collects.
When an aphid feeding on a plant stem, its mouthparts are seen to be taking food from the
phloem.
How We Know It Requires ATP
The companion cells contain many mitochondria
Translocation stops when metabolic poison is used
Rate of flow of sugars is too quick for diffusion alone
How We Know It Uses This Mechanism
The pH of the companion cells is higher than that of surrounding cells
Concentration of sucrose higher in the source than in sink
Evidence Against Mechanism
Not all solutes in the phloem sap move at same rate
Sucrose move at same rate rather than going to area with low concentration a lot more
quickly
The role of sieve plates is unclear…read more

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